A high resolution structure (1.9 E) of active Chymase complexed with PMSF has recently been solved. A similarly high resolution structure of Pro-Chymase would provide insight into the processing of this important serine protease. Such a structure would address issues of conformational changes involved in activation and the method by which the two residue pro-region of the enzyme renders the enzyme inactive. Crystallization conditions have been developed for Pro-Chymase, but as yet yield only small crystals (0.2 mm x 0.05 mm x 0.02 mm). In house collection has resulted in data to 3.5 - 4.0 E. This data indicates cell dimensions of 70 x 70 x 117 in the p3121 space group. Further refinement of crystallization conditions is ongoing as well as efforts to co-crystallize the protein with heparin. In vivo heparin has been shown to associate with Pro-Chymase and is important in the activation of the protein by dipeptidylpeptidase I. A co-crystal structure would elucidate the mechanism by which heparin presents and coordinates the highly specific cleavage of the dipeptide proregion of chymase by the non-specific dipeptidylpeptidase I.